1. Technical Field
The present invention relates generally to semiconductor devices, and more particularly, to a silicon-on-insulator based radiation detection device and method of detecting ionizing radiation.
2. Related Art
Ionizing radiation can cause integrated circuits (IC) to malfunction. Accordingly, the ability to detect ionizing radiation is a key attribute of current semiconductor device technologies. Achieving this detection, however, is becoming increasingly difficult as further miniaturization continues. In particular, the continued miniaturization of the interface between gate and channel, i.e., the gate dielectric or gate oxide, results in smaller and harder to detect signals.
In silicon-on-insulator (SOI) technology, and particularly partially-depleted SOI technology, front gate radiation detection is offered. FIG. 1 shows an illustrative SOI-based semiconductor device 10 including a front gate 12 and back gate 14. “Front gate” indicates a typical transistor structure, while “back gate” refers to a gate in which the silicon substrate forms the gate and a buried insulator layer forms a gate dielectric with source/drain diffusion regions of the front gate. For example, device 10 includes a substrate 20, a buried insulator layer 22 (i.e., BOX for buried oxide) over substrate 20 and an active layer 24 formed over buried insulator layer 22. “Front gate” 12 is provided in the form of a field-effect transistor (FET) over active layer 24 and includes, inter alia, a gate 30, a gate dielectric 32 and source/drain diffusion regions 34. “Back gate” 14 is formed below buried insulator layer 22 and includes part of buried insulator layer 22 acting as a gate dielectric 36 and part of silicon substrate 20 acting as a gate 38. A channel region 40 is formed between front gate 12 and back gate 14. The active part of channel region 40 extends from source to drain. Front gate 12 is referred to as a “strong gate” because it has higher capacitive coupling to channel region 40, and back gate 14 is referred to as a “weak gate” because it has reduced capacitive coupling to channel region 40.
Radiation causes trapped charge in the insulating regions (layer 22 in FIG. 1). This trapped charge alters the device threshold voltage and causes it to leak. The thicker the dielectric layer the stronger the signal, i.e., more leakage per unit trapped charge. General device performance scaling however drives gate dielectric 32 to be as thin as possible. However, the thicker back gate 14 (weak gate) has less control over device leakage due to the presence of the front gate 12 (strong gate). That is, since front gate 12 is the stronger gate, it controls whether current flows through channel region 40. Since back gate 14 typically has a thicker gate dielectric 36, i.e., the buried insulator layer, it provides more sensitive structure to detect ionizing radiation. Unfortunately, because the front gate controls channel region 40, the back gate signal is dominated by the stronger front gate, making radiation detection by the back gate impossible.
In view of the foregoing, there is a need in the art for a semiconductor structure for SOI technology with improved back gate detection.